Browsing by Author "Dr. Robert Kolbas, Committee Member"

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A 1 Mbps Underwater Communication System Using a 405 nm Laser Diode and Photomultiplier Tube
(2008-12-07) Cox, William Charles, Jr.; Dr. Brian Hughes, Committee Member; Dr. John Muth, Committee Chair; Dr. Robert Kolbas, Committee Member
Radio frequency communications in seawater are impractical due to high conductivity of seawater limiting the propagation of electromagnetic waves. Current methods, such as acoustic communication, are limited in bandwidth, and the use of cables, such as fiber optic, are expensive and not practical for autonomous vehicles. Underwater tethered communication systems are also very costly to repair if damaged. Optical wireless communications that exploit the blue/green transparency window of seawater potentially offer high bandwidth, although short range, communications. The goal of this Masters thesis was to build sufficient infrastructure to experimentally validate the performance of underwater optical communication systems under laboratory, but hopefully realistic, water conditions. An optical transmitter based on a 405nm blue laser diode was constructed. The transmitter is capable of sourcing 200mA of current to a blue laser diode at speeds of up to 200MHz. The receiver was based on a photomultiplier tube. The high gain and blue/green sensitivity of a photomultiplier tube make it ideal for underwater optical communications. Finally, a 1,200 gallon water tank was constructed that allows the water conditions to be appropriately controlled to simulate an ocean environment Experiments were conducted to validate the design and construction of the receiver, transmitter and water tank. An underwater optical data link was demonstrated that was capable of transmitting data at 500kpbs in return-to-zero format, or 1Mpbs in non-return-tozero format. The transmitted signal could then be optically detected, digitized and stored on a PC for later signal processing.
Investigation of Surface States in Gallium Nitride Devices using a New High Frequency Measurement Technique
(2006-10-05) Ramachandran, Ramya; Dr. Douglas Barlage, Committee Chair; Dr. Robert Kolbas, Committee Member; Dr. Mark Johnson, Committee Member
Surface states place a limitation on the high-frequency behavior of Gallium Nitride devices by causing RF dispersion. They are also a source of undesirable 1⁄f noise. This thesis specifically aims to explore techniques to address known experimental observations of dispersion phenomena of unknown origin in the 1-30 GHz frequency range. A new method to investigate these at high frequencies is proposed. It involves the measurement of S-parameters between the drain and source of a GaN nin structure in the GHz frequency range. The basis of the proposed technique is the assumption that the behavior of surface states and other dispersion phenomena can be isolated by subtracting the behavior of the device from the measured Y-parameters.
Laser Molecular Beam Epitaxial Growth and Properties of III-Nitride Thin Film Heterostructures on Silicon
(2006-03-01) Rawdanowicz, Thomas Adolph; Dr. Gerd Duscher, Committee Member; Dr. Robert Kolbas, Committee Member; Dr. J. Michael Rigsbee, Committee Member; Dr. J. Narayan, Committee Chair
The principal goal of this research was the investigation and process development of epitaxial growth mechanisms for the direct depositions of heteroepitaxial GaN thin films directly on Si(111) and Si(100) substrates without the incorporation or the formation of an interlayer at the GaN/Si interface. The research involved the design, development and implementation of a physical vapor deposition system based on a laser ablation process in an ultra high vacuum environment. Consideration is given to the role and control of substrate temperature as a function of elapsed deposition process time and its influence on lowering interfacial energies and limiting silicon nitride interlayer formation. The research results show that crack-free growth of 2 μm thick heteroepitaxial AlN and GaN thin films on Si(111) substrates can be achieved without the use of interlayer films. These thin film depositions resulted in atomically clean and chemically abrupt interfaces, while restricting the formation of silicon nitride at the interface. The resulting AlN and GaN epitaxial relationship on Si(111) is confirmed as [0002]║Si[111], [2110]║Si[110], and [0110]║Si[211]. The III-Nitride thin film on Si(111) is established by domain matching epitaxy (DME) exhibiting a ratio of (2110):(110) interplanar distances of 6:5 for GaN:Si and 5:4 ratio for AlN:Si with clean interfaces along silicon nitride free terraces of the Si(111) surface. Moreover, variations in the domain matching epitaxy were observed to result in the further reduction of residual interfacial strain with the incorporation of domain matching ratios of 5:4 for GaN/Si(111) and 6:5 for AlN/Si(111) occurring with a calculated frequency of nine 5:4 ratios for each 6:5 plane matching ratio for GaN/Si(111) and two 6:5 ratios for each 5:4 plane matching ratio for AlN/Si(111). For the case where silicon nitride (SiNx) is allowed to form at the interface, elemental analysis using electron energy loss spectroscopy provided evidence that the formation of SiNx occurs as a result of subsequent nitrogen diffusion to the GaN/Si interface after the GaN epitaxy is established.
A Prototype Hadamard Imaging System
(2006-09-07) Fothergill, Daryl William; Dr. John Muth, Committee Chair; Dr. Robert Kolbas, Committee Member; Dr. David Lalush, Committee Member
The purpose of this thesis was to investigate the possibility of creating an inexpensive imaging system that would be suitable for imaging small animals, either the skin of mice for skin cancer studies, or potentially whole animal imaging. In this optical system the light is collected by an array of 31 optical fibers. In more advanced systems one can envision 1024, or even more fibers being used to increase the resolution of the image. The principle novelty of this system is that Hadamard encoding enabled only one photodetector to be used for the whole system rather than one detector for each fiber. There are two important advantages that can be obtained by using this strategy. First, especially with large numbers of fibers, the overall signal to noise ratio of the system can be improved. Second, the cost and complexity of the system can be greatly reduced. In cases where the signal to noise ratio is low, such as fluorescence detection, designing a system that has only one detector has substantial advantages. This system can also be applied to other sensor applications with large numbers of inputs. To our knowledge Hadamard imaging has not been applied to macroscopic imaging applications, or to small animal imagining. Plastic fiber optics are used to gather and pixilate the spatially dependent inputs from the light source. The optical fibers were then switched on and off using a rotating mask encoded with a Hadamard matrix by drilling holes in the mask. The encoded light was then detected with an inexpensive photodetector and decoded using a desktop computer. The system is automated by using a BASIC Stamp to control the stepper motors and LabVIEW. Future improvements such as a stationary MEMS mask and glass optical fibers that could improve the system by making it more efficient and smaller in size are discussed.